MIMO is an abbreviation for Multiple-Input Multiple-Output and refers to a method of improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas, instead of a conventional method employing one transmission antenna and one reception antenna. In other words, MIMO technology allows a transmitter or a receiver of a wireless communication system to use multiple antennas, so that capacity or performance can be improved. Here, MIMO refers to multiple antennas.
In order to receive a message, MIMO technology is not dependent on a single antenna path. Instead, MIMO technology applies a technique of completing a whole message by gathering data fragments received via several antennas. Since MIMO technology can improve a data transmission rate in a specific range or increase system range at a specific data transmission rate, it is next-generation mobile communication technology which can be widely used in mobile communication terminals, relays, etc. Attention is being paid to this technology as a next-generation technology capable of overcoming limitations in mobile communication transmission capacity that has become a critical situation due to expansion of data communication.
FIG. 1 shows the configuration of a general MIMO communication system. As shown in FIG. 1, if the numbers of transmission and reception antennas are simultaneously increased to NT and NR, respectively, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case in which only either a transmitter or a receiver uses multiple antennas. Accordingly, it is possible to increase transmission rate and to remarkably improve frequency efficiency. Theoretically, the transmission rate according to an increase in channel transmission capacity can be increased by an amount obtained by multiplying an increase rate Ri indicated in the following Equation 1 by a maximum transmission rate Ro in case of using one antenna.Ri=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system using four transmission antennas and four reception antennas, it is possible to theoretically obtain a transmission rate which is four times the transmission rate of a single antenna system. After an increase in the theoretical capacity of the MIMO system was first proved in the mid-1990s, various techniques for substantially improving data transmission rate have been actively developed. Several of these techniques have already been incorporated in a variety of wireless communication standards such as the 3rd generation mobile communication and the next-generation wireless local area network.
Active research up to now related to the MIMO technology has focused upon a number of different aspects, including research into information theory related to the computation of MIMO communication capacity in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of a MIMO system, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
A communication method of the MIMO system will be described in more detail using a mathematical model. As shown in FIG. 1, it is assumed that NT transmission antennas and NR reception antennas are present. In transmitted signals, if NT transmission antennas are present, the number of pieces of maximally transmittable information is NT. The transmitted information may be expressed as vectors as indicated by the following Equation 2.s=└s1,s2, . . . ,sNT┘T  [Equation 2]
The transmitted information s1, s2, . . . , sNT may have different transmit powers. If the respective transmit powers are P1, P2, . . . , PNT, the transmitted information with adjusted powers may be expressed as follows.ŝ=[ŝ1,ŝ2, . . . , ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Equation 3]
In addition, ŝ may be expressed using a diagonal matrix P of the transmit powers as follows.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          S                      1                                                                                                                                  S                      2                                                                                                            ⋮                                                                                                              S                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
Consider that NT actually transmitted signals x1, x2, . . . , xNT are configured by applying a weight matrix W to the information vector ŝ with the adjusted transmit powers. The weight matrix W serves to appropriately distribute the transmitted information to each antenna according to a transport channel state, etc. The transmitted signals x1, x2, . . . , xNT may be expressed by using a vector X as indicated by the following Equation 5 in which wij denotes a weight between an i-th transmission antenna and j-th information. W is also called a precoding matrix.
                                                        x              =                            ⁢                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                            ⁢                                                                                     [                                                                                                                        w                            11                                                                                                                                w                            12                                                                                                    …                                                                                                      w                                                          1                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                                                      w                            21                                                                                                                                w                            22                                                                                                    …                                                                                                      w                                                          2                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            w                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      w                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                                                          N                                T                                                            ⁢                              1                                                                                                                                                            w                                                                                          N                                T                                                            ⁢                              2                                                                                                                                …                                                                                                      w                                                                                          N                                T                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                                                      s                              ^                                                        1                                                                                                                                                                                                          s                              ^                                                        2                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                        j                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                                                      N                              T                                                                                                                                            ]                                                                                                                          =                            ⁢                              W                ⁢                                  s                  ^                                                                                                        =                            ⁢              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
If NR reception antennas are present, respective received signals y1, y2, . . . , yNR of the antennas are expressed as follows.y=[y1,y2, . . . ,yNR]T  [Equation 6]
If channels are modeled in the MIMO radio communication system, the channels may be distinguished according to transmission/reception antenna indexes. A channel from the transmission antenna j to the reception antenna i is denoted by hij. In hij, it is noted that the indexes of the reception antennas precede the indexes of the transmission antennas in view of the order of indexes.
The channels may be combined and expressed in the form of a vector and a matrix. An example of the expression of the vector will be described. FIG. 2 is a diagram showing channels from the NT transmission antennas to the reception antenna i.
As shown in FIG. 2, the channels from the NT transmission antennas to the reception antenna i may be expressed as follows.hiT=[hi1,hi2, . . . ,hiNT]  [Equation 7]
Additionally, all channels from the NT transmission antennas to the NR reception antennas through the matrix indicated by the above Equation 7 may be expressed as follows.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Additive White Gaussian Noise (AWGN) is added to the actual channels after a channel matrix H. The AWGN n1, n2, . . . , nNR added to the NR reception antennas may be expressed as follows.n=[n1,n2, . . . ,nNR]T  [Equation 9]
Through the above-described mathematical equations, the received signals may be expressed as follows.
                                                        y              =                            ⁢                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R                                                                                                                    ]                                                                                        =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                            ⁢                              Hx                +                n                                                                        [                  Equation          ⁢                                          ⁢          10                ]            
Meanwhile, the number of rows and columns of the channel matrix H indicating the channel state is determined by the number of transmission and reception antennas. The number of rows of the channel matrix H is equal to the number NR of reception antennas and the number of columns thereof is equal to the number NT of transmission antennas. That is, the channel matrix H may be expressed by an NR×NT matrix. Generally, the rank of the matrix is defined by the smaller of the number of rows or the number of columns, which are independent of each other. Accordingly, the rank of the matrix is not greater than the number of rows or columns. The rank of the channel matrix H is expressed as follows.rank(H)≦min(NT,NR)  [Equation 11]
In a MIMO system, precoding provides beamforming gain and diversity gain to a transmitter and a receiver, thereby offering high system throughput. However, a precoding scheme should be appropriately designed in consideration of antenna configuration, channel environments, system architecture, etc.
Generally, a MIMO system performing precoding in order to minimize complexity and control signaling overhead uses a codebook based precoding scheme. The codebook includes a predetermined number of precoding vectors/matrices which is determined between the transmitter and the receiver according to a transmission rank and the number of antennas. The transmitter performs precoding upon a signal to be transmitted by selecting a specific precoding vector/matrix within the codebook according to channel state information received from the receiver. In some cases, the transmitter may perform precoding by selecting a precoding matrix according to a predetermined rule without receiving the channel state information from the receiver and may transmit the precoded signal.
A channel estimation method in a MIMO transmission system using precoded spatial multiplexing will now be described. For channel estimation, a Reference Signal (RS) is provided. If a non-precoded RS is used, each channel from transmission antenna ports is estimated from the RS and an equivalent channel can be obtained by a combination of an estimated channel matrix and a precoding weight. Meanwhile, if a precoded RS is used, since the precoded RS has been precoded by the same precoding weight as in data transmission, an equivalent channel can be directly obtained from the precoded RS. An equivalent channel matrix is composed of channel vectors experienced by each layer and each channel vector can be estimated by a Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), or Code Division Multiplexing (CDM) scheme.
In a MIMO transmission scheme of rank-2 or more, precoded RSs for two or more layers may be transmitted through one resource element (time and frequency resources) by performing multiplexing using a CDM scheme. In this case, CDM resources corresponding to the number of transmission ranks (or number of transmission layers) are needed. That is, for rank-2 transmission, two CDM resources are needed, and for rank-3 transmission, three CDM resources are needed.
When more CDM resources are used as rank increases, transfer overhead of control information for indicating the resources increases. For example, a DeModulation Reference Signal (DMRS) for uplink transmission can be multiplexed by using a Cyclic Shift (CS) as CDM resources and the CS value for the DMRS is indicated to a User Equipment (UE) through a Downlink Control Information (DCI) format. If the CS values which should be indicated to the UE differ according to rank, various DCI formats need to be defined and the UE may bear an increased burden of blind-decoding the DCI format.